The present invention relates generally to the field of probes, apparatus and methods for investigating or examining samples. More specifically, the present invention relates to a probe which comprises a dielectric body which can be used to direct light from the probe onto the sample and/or collect light from the sample into the probe. The present invention also relates to apparatus such as transceivers, which may be used with such probes. The present invention is primarily intended for use in the frequency range 25 GHz to 100 THz. This frequency range is colloquially referred to as the THz range.
There has been much interest in using THz radiation to look at a wide variety of samples using a range of methods. THz radiation has used for both imaging samples and obtaining spectra. Recently, work by Mittleman et al, IEEE General Selective Topics in Quantum Electronics, Volume 2, No. 3, September 1996, page 679 to 692 illustrates the use of THz radiation to image various objects such as flame, a leaf, a moulded piece of plastic and semiconductors.
THz radiation penetrates most dry, non-metallic and non-polar objects like plastics, paper, cardboard and non-polar organic substances. Therefore, THz radiation can be used instead of X-rays to look inside boxes, cases, etc. THz photons are lower energy than those of X-rays and are non-ionising. Therefore, the health risks of using THz radiation are expected to be vastly reduced compared to those using conventional X-rays.
There is a need to develop a probe which can be used to either emit or detect THz radiation. Preferably, the probe is configured to both emit and detect THz radiation.
Previously, probes using THz radiation have included Lai et al Appl. Phys. lett. 69 1843 (1996). In this paper, a photo-conductive-sampling probe is fabricated from low-temperature-grown GaAs. The probe is mounted on a single mode optical fibre. The photo-conductive sampling probe is used to detect radiation. A later paper by Lai et al in Appl. Phys. lett. 72 3100 (1998) discusses a micro-machined photo-conductive THz emitter which is formed from low-temperature-grown GaAs mounted on a pair of single mode optical fibres.
The above two papers demonstrate that it is possible to use a fibre optic THz probe to emit THz or detect THz. However, no consideration is given to what happens to the THz radiation once it is generated or how the THz beam actually arrives at the detector.
Fattinger et al in Appl. Phys. lett. 53 1480 (1998) demonstrates simultaneous emission and detection of a THz beam using a silicon on sapphire photo-conductive device and a solid hemispherical Sapphire mirror. In a slightly later publication, Appl. Phys. lett. 44 490 (1989), the same research group demonstrates transmission of a THz beam from a separate emitter to a separate detector. Both the emitter and detector use solid hemispherical sapphire lenses. However, there is no disclosure of how to direct the beam onto a sample or from a sample.
Although, the field of geometrical optics is well developed for radiation of certain wavelengths, different and extra restrictions apply when developing a probe which can operate with THz radiation. None of the above documents address the problem of the different absorption and propagation characteristics of THz radiation.